Recent WHO reports identify Mycobacterium tuberculosis (Mtb) as the single largest cause of death of individuals infected with the Human Immunodeficiency Virus (HIV), a co-infection state highly prevalent in Sub- Saharan Africa. Susceptibility to TB infection appears to be more complex than loss of surveillance through depletion of CD4+ T-cells. In preliminary experiments we show that replication of Mtb in macrophages is enhanced by co-infection with HIV. We propose extensive cellular and molecular analyses of experimental co- infections to be performed at Cornell University, Ithaca, NY. In addition, we have an HIV study cohort at the Queen Elizabeth Central Hospital (QECH) in Blantyre, Malawi and will probe the relevance of in vitro phenotypes through analysis of macrophages and lymphocytes from the lungs of HIV-infected individuals. This current study is founded on the hypothesis HIV infection of alveolar macrophages plays a significant role in the increased susceptibility to development of active tuberculosis. Specific Aim #1. How does HIV infection promote the intracellular growth of Mtb? A. What causes the increased permissiveness of the host cell? To elucidate the mechanism(s) by which HIV infection promotes bacterial survival we are targeting the following 3 questions: i. Is the Mtb growth advantage restricted to HIV infected cells? ii. Does HIV infection modulate the microbicidal activities of the macrophage? iii. What HIV ORFs mediate this alteration in the macrophage environment? B. How does Mtb exploit this increased permissiveness? We propose both biased and unbiased genetic screens to identify genes required for enhanced Mtb growth in HIV-infected HMDM. i. Biased approach based on the hypothesis that the altered lipid metabolism favors bacterial replication. ii. Unbiased Transposon-insertion site mapping (TraSH) screen of Mtb in HIV-infected macrophages. Specific Aim #2. How does HIV impact the immune environment of the lung to render it more permissive to infection and/or progression of tuberculosis? We will assess the capacity of HIV to render AM more permissive to infection by Mtb. A. Utilization of HMDMs as surrogates for the development of a Mtb survival assay B. How is the functional phenotype of human alveolar macrophage modulated in the context of HIV infection?